1995 Nissan 240sx will not keep running.

The battery is dead. Replaced the alternator. Engine will only start with charger connected to the battery. When you disconnect the charger the engine stops running.
Runs without a problem as long as the battery charger is connected.

an engine running once started, the alternator is supposed to take over for the battery once the engine is running.

We already know the battery is dead.The vehicle will not keep running. The battery does not keep
an engine running once started, the alternator is supposed to take over for the battery once the engine is running.
We already know the battery is dead.

you need to replace your battery.since the alternator is supposed t o charge the battery. y dont you start your vehicle, have it run for 1 hour or go on an hours drive, come back home, turn it off,after five minutes start the vehicle..see if it starts.if it does not, that means your battery is dead.

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Check to make sure the power lead from the alternator to the battery has a good connection. Sometimes the connectors get corroded, or the wire separates inside the insulation. You can check the wire for continuity with an Ohm meter...make sure to disconnect the wire at both ends when you use the Ohm meter.
Make sure the battery has a good clean ground. Make sure the alternator is grounded to the engine and the engine ground strap is intact and has clean grounds on both the engine and frame. Test the alternator with the engine running using a voltmeter across the battery. You should read 13.5 to 14.7 volts. If the alternator is not charging the battery will go dead under normal operation. If you don't get the correct charging voltage either your connections are bad or the alternator is bad.

Put a voltmeter on the battery with the engine running. It should read at least 13.5 volts to know the alternator is working good. If it reads below 13 volts, it is not working good, may not be working at all.
What cab electrical circuits are not working? Or are they working now?

With engine running used a voltmeter and check voltage at battery, you will be checking alternator out put voltage, should have a reading of no less then 13.0 volts, if less bad alternator, the reason it keeps going dead is that the car is using the battery to run it .till the battery has no more power, then you jump it and the battery is charge up again and will run again till it goes dead again.

Check the alternator and battery clemmas/wires coming from alternator. It looks like no charge from it coming to battery. If it is second battery already this is the sign. You can do it free of charge at autozone store. But any way as a temporary solution you can buy battery charger ($25 walmart) and charge the battery back if battery is damaged you will see it right away (it wont be charging). After you charge battery back start the engine run it for 30 minutes (if Alternator OK it is enough to charge battery back), stop and start again. If no start alternator is a problem.
Good Luck

My Nissan Primera EGT (1993 Reg'd) has an immobiliser, which was causing a similar problem. Turned on ignition and nothing. The immobilizer was faulty to the starter motor, so we disconnected it from the starter and left the immobilizer on the fuel line. Starts now, no problem and the immobilizer would still stop the potential thieves as they'd have no idea without the immobilizer key why the engine will no more than start then stutter to a big full stop!!.The 240SX is probably a more modern Nissan but I bet there's an immobilizer built into it somewhere. You may need to take it to a Nissan expert or Dealer to check out. It's almost definitely something weird like an immobilizer or maybe the starter motor's blown it, but then you'd have heard the graunching noises that starter motors make when they're about to clap out, they don't usually just go. There may also be some disconnection of wires to the alternator, which amplifies the spark to the engine, but the starter motor would still turn, your engine just wouldn't fire up with the too weak spark. So if nothing moves, it's most likely an immobilizer or possibly (but unlikely) a sudden death starter motor.